This application claims priority to International Application No. PCT/DE00/01167 which was published in the German language on Feb. 15, 2001.
The invention relates to a method for operating an RF power amplifier, particularly a mobile radio transmitting output stage, according to the preamble of claim 1, and an arrangement for carrying out this method.
Mobile voice and data communication is a technical field which is developing extremely rapidly and has a very wide economic perspective. Utilization of this perspective presupposes the continuously improved utilization of the available frequency resources and the opening up of possibilities for transmitting ever increasing volumes of data with the required reliability.
When new standards are established which must take into account these requirements, the use of modulation methods with an inconstant envelope curve, for example of combined phase and amplitude modulation methods, is defined, differently from the current GSM standard, for example. Thus, the use of QPSK (Quadrature Phase Shift Keying) modulation, as an example of such a modulation method, is provided for the UMTS (Universal Mobile Telecommunication System) standard which determines the near future of mobile technology.
RF signals modulated in this manner must be amplifiedxe2x80x94for example in the transmitting output stage of a mobile radio terminalxe2x80x94with high accuracy and without distortion, i.e. free of amplitude distortion due to non-linearities of the amplifier characteristic and free of phase distortion which is caused by the AM-to-PM conversion process. This is because the digital modulation methods defined in the mobile radio standards of the future, for example the abovementioned UMTS standard, are extremely sensitive to distortion-related bit errors. The system standards or specifications, therefore, contain strict rules with regard to the permissible amplitude and phase errors and the permissible adjacent channel power (ACP).
The error limits and adjacent channel powers specified in the specifications can only be maintained with high-quality linear transmit amplifiers or by using additional equalizing circuits for linearizing the transmission characteristic. The linear operation of a Class A amplifier presupposes that the 1-dB output compression point of the amplifier is clearly above the peak power level of the output signal. This requirement must be taken into consideration in the design of all amplifier stages and bias currents. Such a design of the amplifier for maximum output power to be provided results in an unnecessarily high current consumption at low power levels. The efficiency of the amplifier thus drops rapidly with decreasing output power level. This presents a serious utility value disadvantage for the user because it entails an unnecessarily high current consumption and thus relatively short charging cycles for the battery of the mobile radio terminal. This is particularly serious in future devices according to the UMTS standard because the transmitter of these devices is continuously operated in so-called full duplex mode.
Additional known measures for linearizing the transmission characteristic of an amplifier arrangement which is non-linear per se include, for example, a pre-emphasis of the modulated signal by means of which the non-linear transmission characteristic of the actual amplifier is replicated inversely so that the superposition of the actual transmission characteristic of the amplifier and the inversely replicated amplitude response of the modulated signal results in an approximately linear transfer function overall. Like other measures of a similar type, this measure, however, is relatively expensive to implement electronically and only provides for a real linearization of the overall transmission characteristic up to a certain degree.
It is also known to compare the current consumption of individual transistor stages with a nominal quantity which depends on the expected-output power and to correct the transistor operation points in dependence on the result of the comparison, with the aim of linearizing an amplifier characteristic.
To suppress any unwanted side effect of non-linearities of the amplifier characteristic of the transmit amplifierxe2x80x94namely the widening of the transmit frequency spectrum by intermodulation effects (mainly of the third order)xe2x80x94it is also known to separate the intermodulation products from the total signal as an error signal by means of a comparison of input and output signal and by suitable amplitude and phase adjustment at the output of the comparator. The error signal thus ascertained can then be applied via suitable coupling elements to the input of the amplifier to be linearized in order to achieve intermodulation feedback (or to its output in order to achieve a feed forward). However, this is an additional measure which can only minimize an effect of the abovementioned non-linearities.
In one embodiment of the invention there is a method for operating an RF power amplifier with an approximately linear characteristic. The method includes, for example, adjusting at least one operating parameter of the amplifier or at least one active component of the amplifier, and controlling a minimum collector or drain current of the active component, or an amplifier stage and an input signal level of the RF power amplifier, based on a quotient of harmonic power and useful-signal power of the amplifier output signal.
In another aspect of the invention, the input signal level is controlled as power control by means of a control loop.
In another aspect of the invention, the input signal level is controlled with access to an allocation rule, between amplification or output signal level values and input signal level values by an input level control unit.
In yet another aspect of the invention, the input level control unit operates based on externally transmitted amplification or output signal level values or amplification or output signal change commands.
In another aspect of the invention, the method includes varying an attenuation of a preceding attenuation section and/or the amplification of a preceding small-signal amplifier for controlling the input signal level.
In another embodiment of the invention, there is an RF power amplifier arrangement, with an approximately linear characteristic. The arrangement includes a bias adjusting unit to adjust a minimum collector or drain current of at least one active component or of an amplifier stage, and an input signal level adjusting unit to adjust the input signal level based on a quotient of harmonic power and useful-signal power of the amplifier output signal.
In another aspect of the invention, the bias adjusting unit comprises a bias control loop and the input signal level adjusting unit comprises a power control loop which is substantially separate from the bias control loop.
In another aspect of the invention, the input signal level adjusting unit includes an input signal level control unit with a storage unit in which an allocation rule between amplification or output signal level values and input signal level values is stored.
In still another aspect of the invention, the input signal level adjusting unit includes a controllable attenuation section or a controllable linear small-signal amplifier.
In another aspect of the invention, the bias adjusting unit includes a high-pass filter and a first power detector, which follows the filter at least indirectly, and the input signal level adjusting unit includes a low-pass filter and a second power detector, which follows the filter at least indirectly.
In another aspect of the invention, the bias adjusting unit, and the input signal level adjusting unit each includes a temperature compensation element.
In yet another aspect of the invention, the bias adjusting unit and/or the input signal level adjusting unit include a reference value providing unit and a comparator or subtraction device which are connected to the comparator or subtraction device latter via an input.